In the current steel industry, blast furnace-converter processes are widely used and predominant. In a blast furnace process, iron ore, which is a main starting material, is reduced by blowing high-temperature heated air into it, using coke as a reductant, thereby obtaining carbon-saturated molten iron, which is called molten pig iron.
Other known processes for producing molten pig iron include DIOS, FINEX, and SMP (Scrap Melting Process). Since reduced iron obtained by reducing iron ore with natural gas or coal is generally obtained in a solid state, it is used as a secondary iron material in a converter process or an electric arc furnace process.
In general, molten pig iron includes sulfur, carbon, and phosphorus in amounts harmful to steels. Particularly, Cu contained in scrap iron cannot be removed in SMP, which causes various disadvantages.
Of these, regarding sulfur, desulfurization treatment is performed as a treatment prior to converter refining because a desulfurization reaction is likely to proceed under a reduced atmosphere, as the temperature is high. Thereafter, molten pig iron that has been subjected to desulfurization treatment is supplied into a converter; and supersonic pure oxygen gas is blown from above using a metal lance whose exterior has been water-cooled, or pure oxygen gas is blown from an oxygen gas inlet provided at the bottom of the converter, or a method including both steps is used to cause a decarburization reaction, thereby adjusting the amount of carbon and the temperature to desired levels. Regarding phosphorous, although dephosphorization treatment has recently been performed as a step prior to converter refining, the use of burned lime in converter refining allows for the relatively easy removal of phosphorous.
The essential role of converter refining is to adjust the amount of carbon to a desired level by a decarburization reaction and to control temperature to ensure smooth operation in the subsequent step. A decarburization reaction by pure oxygen gas is an exothermic reaction; the temperature of molten iron rises along with the development of the decarburization reaction, and the temperature may exceed a desired level. In such a case, scrap iron or the like is generally used; the melting heat of scrap iron is utilized to prevent the temperature from becoming excessively high. Examples of materials used for inhibiting such temperature rise, e.g., coolants, include iron ore, limestone, and the like, in addition to scrap iron.
In such a conventional converter process, one known problem is that a large amount of converter dust is generated. The generation of converter dust causes loss of sensible heat that is brought out of the converter by dust, and economic loss such as dust disposal cost, as well as about 3% iron yield loss during converter refining. Therefore, dust generation reduction has been an object in the converter process.
It is known that converter dust is generated by the following three phenomena.
1) When pure oxygen gas is blown into molten pig iron in a converter, the temperature of the pure oxygen gas blown into the molten pig iron suddenly rises from about 300K to about 1770K. Therefore, the pure oxygen gas expands to roughly about 6 times its initial volume. The pure oxygen gas is reacted with carbon in the molten pig iron while floating in the molten pig iron, and converts to CO gas having a double volume. The CO gas bubbles float in the molten pig iron while increasing their temperatures by the reaction heat of carbon in the molten iron and pure oxygen gas at an atmosphere temperature of about 1770K, and then burst when escaping from the surface of the molten iron. Since the CO bubbles rise up to the surface and burst while expanding, large amounts of molten iron splashes are scattered in an atmosphere. The splashes are removed from the converter together with exhaust gas, and collected as converter dust in a dust collector. Although some of the molten iron splashes are oxidized with oxygen in an atmosphere, those having a large particle size are oxidized only at the surface, and the inside is present as iron and collected as so-called coarse particle dust. The converter dust is called bubble-burst dust, and the generation of bubble-burst dust increases in proportion to the amount of decarburization by pure oxygen gas.2) When in contact with molten iron, pure oxygen gas is reacted with carbon or iron to form a relatively high-temperature region called a firing point. The temperature of the firing point is said to be a high temperature exceeding the boiling point of iron, i.e., 2750° C. In this firing point region, iron vapor is generated, and removed from the converter together with exhaust gas. The iron vapor is oxidized by oxygen in an atmosphere to become a finely divided iron oxide called fume dust. This fume dust is collected by a dust collector, and used again as a starting material.3) When a pure oxygen gas jet collides with molten pig iron, a concave is formed on the surface of the molten pig iron depending on collision conditions, and some molten iron particles are blown off by a gas jet that flows along the concave. This phenomenon is called spitting, and dust generated by this phenomenon is called spitting dust. Most spitting dust is coarse particles, and collected as coarse particle dust.
Various methods are suggested to inhibit the generation of converter dust. For example, Patent Literature 1 discloses optimizing the angle, diameter, and position between axial center sides of two or more cyclic gas nozzles concentrically disposed at a top-submerged lance to smooth a gas jet ejected from a lance in the circumferential direction and the radial direction, thereby reducing dust resulting from spitting.
Patent Literature 2 to 6 disclose methods for preventing spitting dust or fume dust. In these methods, since the amount of decarburization by pure oxygen gas is not reduced, reduction in bubble-burst dust cannot be expected. To reduce spitting dust, soft blowing of an oxygen jet is effective. As described in the aforementioned patent literature, various methods are known in which the shape of a jet is changed, the distance between a lance and the surface of molten pig iron is increased to keep an appropriate colliding force of an oxygen jet, two or more oxygen jet outlets are provided and a suitable jet angle is determined, slag with a low viscosity is formed at an early stage and spitting dust is obtained by the slag, etc. Regarding a method for preventing fume dust, in principle, fume dust can be prevented by reducing the firing point; however, the use of material only for reducing the firing point results in energy loss. Accordingly, this method is generally not employed.
Thus, the prevention of converter dust involves the essential part of converter refining; this is a problem that, thus far, remains unsolved.